This invention relates in general to devices for comparing an optical frequency to a radio frequency and more particularly to an improved device for establishing a rational ratio N/M to be established between an optical frequency and a radio frequency (RF).
In U.S. Pat. No. 4,631,498 to applicant, a wave meter/frequency locking technique is disclosed which is suitable for indirectly locking an optical frequency f to a radio frequency f' or for locking the radio frequency to the optical frequency. The indirect locking is accomplished by using a filter which has a characteristic frequency f.sub.f. A beam of optical frequency f is phase modulated by a subcarrier signal of average frequency f' that is itself modulated at frequency f". The modulated beam is passed through the filter to a detector to produce a detector output signal that has frequencies at linear integral sums of f' and f". A pair of control signals are generated that are proportional to the amplitude of the two components of the detector output signal. These control signals are used separately in a pair of servo loops to separately establish fixed values of f/f.sub.f and f'/f.sub.f. The value of f/f.sub.f is stepped to a different value.
By measuring f'/f" at each of these values of f/f.sub.f, the value of f can be determined.
In the frequency locking technique described in U.S. Pat. No. 4,631,498, the filter used is an optical cavity. The control signals in the servo loops establish the two frequency ratios. This is performed by locking through an optical servo loop the optical frequency f to a maximum of the resonant mode structure of the optical cavity reflectance and locking through a RF servo loop the first-order sidebands to the minima of the resonant mode structure, where the minima may or may not be adjacent to the maximum. In other words, the control signals in the servo action set the subcarrier frequency f' equal to an odd half integral multiple of the optical cavity's free spectral range. Since the subcarrier frequency f' is used as the reference in deriving the control signals, the detector amplifier and mixers used to derive the control signals must be capable of operating at frequencies of the order of the subcarrier frequency. Since the subcarrier frequency or the RF frequency is of the order of hundreds or even thousands of megahertz, the detector amplifier and mixers used in the circuit must be capable of operating at such high frequencies which is undesirable. For this and other reasons elaborated below the frequency locking technique of U.S. Pat. No. 4,631,498 is not entirely satisfactory. It is therefore desirable to provide an improved frequency locking technique where such undesirable features are alleviated.